No, I take that back: we _do_ have a mechanism for suppressing reads

No, I take that back: we _do_ have a mechanism for suppressing reads
from forwarding data sources which will be good enough to last until
we close the socket, in the form of the override_throttle() functions.
So this finishes up the work in r9283, by manufacturing outgoing EOF
in response to incoming CLOSE on all channel types.

[originally from svn r9284]
[r9283 == c54e228d0497409d72f336ea43f821b736a5ab29]